The present invention relates to an apparatus for compressing gaseous refrigerant for use in a refrigeration circuit of a liquefaction plant.
U.S. Pat. No. 4,698,080 discloses a liquefaction plant of the so-called cascade type having three refrigeration circuits operating with different refrigerants, propane, ethylene and methane. In the first two of these refrigeration circuits the natural gas is pre-cooled, and in the third refrigeration circuit the natural gas is liquefied.
In the first two refrigeration circuits, the propane circuit and the ethylene circuit, the refrigerant is compressed in an apparatus for compressing gaseous refrigerant to a refrigeration pressure and supplied to three heat exchangers in series, wherein in each heat exchanger the refrigerant is allowed to evaporate at a lower pressure in order to remove heat from the natural gas feed. The refrigerant is allowed to partly evaporate in the first heat exchanger at high pressure. The vapour part of the refrigerant at high pressure leaving the first heat exchanger is returned to the compression apparatus and the remaining liquid is allowed to partly evaporate at intermediate pressure in the second heat exchanger. The vapour part of the refrigerant at intermediate pressure leaving the second heat exchanger is returned to the compression apparatus and the remaining liquid is allowed to evaporate at low pressure in the third heat exchanger. The refrigerant at low pressure leaving the third heat exchanger is returned to the compression apparatus.
The third refrigeration circuit, the methane circuit, differs from the other two. A difference is that the natural gas that has been pre-cooled at liquefaction pressure is liquefied in a main heat exchanger by indirect heat exchange with natural gas. The natural gas used for liquefaction is obtained downstream of the main heat exchanger. Downstream of the main heat exchanger, the pressure of the liquefied natural gas is let down in three stages in order to enable storing liquefied natural gas at atmospheric pressure. The three stages yield three streams of gaseous natural gas. The three streams of natural gas used for liquefying the natural gas are compressed in a compression apparatus to liquefaction pressure and returned to the natural gas feed upstream of the main heat exchanger.
The compression apparatus used in the propane circuit is a single compressor comprising three sections. The compressor has a main inlet, two side inlets and one outlet for refrigerant at refrigeration pressure. The main inlet is the inlet for refrigerant at low pressure, the first side inlet is the inlet for refrigerant at intermediate pressure and the second side inlet is the inlet for refrigerant at high pressure.
The compression apparatus used in the ethylene circuit comprises two compressors in series, a first compressor having two sections and a second compressor having one section. The first compressor has a main inlet, a side inlet and one outlet for refrigerant at high pressure, wherein the main inlet is the inlet for refrigerant at low pressure and the side inlet is the inlet for refrigerant at intermediate pressure. The second compressor, having only one section, has a main inlet for refrigerant at high pressure and an outlet for refrigerant at refrigeration pressure. The first and second compressor are interconnected.
The compression apparatus used in the methane circuit comprises three compressors in series, wherein each compressor consists of a single section.
An alternative to the cascade-type liquefaction plant is the so-called propane-precooled multicomponent refrigerant liquefaction plant. Such a plant has a multi-stage propane pre-cooling circuit that is of the kind as described above with reference to the first two refrigerant circuits. In stead of propane, the multi-component refrigerant can be pre-cooled by multicomponent refrigerant. An example of such a plant is disclosed in U.S. Pat. No. 5,832,745. The apparatus for compressing the multi-component refrigerant is also a three-section compressor.
The amount of cooling provided per unit of time in the refrigeration circuit is proportional to the mass flow rate of the refrigerant that is circulated through the refrigeration circuit. With increasing amounts of natural gas to be liquefied the mass flow rate of the refrigerant has to increase. Although an increasing mass flow rate does not affect the number of impellers, it has an effect on the size of the impellers, on the diameter of the housing, and on the inlet velocity into the impellers. Because the latter variables increase with increasing flow rate, an increasing flow rate will result in a larger compressor and higher inlet velocities. Moreover, increasing the diameter of the housing of the compressor requires a thicker wall of the housing. Consequently the compressor is more difficult to manufacture and more difficult to handle.
It is an object of the present invention to provide an apparatus for compressing gaseous refrigerant that overcomes this drawback.
To this end the present invention provides an apparatus for compressing gaseous refrigerant for use in a refrigeration circuit of a liquefaction plant, which refrigeration circuit has an inlet for refrigerant at a refrigeration pressure, a first outlet for gaseous refrigerant at a low pressure, a second outlet for gaseous refrigerant at an intermediate pressure and a third outlet for gaseous refrigerant at a high pressure, which apparatus comprises according to the present invention a first compressor and a second compressor, wherein the first compressor has a main inlet for receiving the refrigerant from the first outlet, a side inlet for receiving the refrigerant from the third outlet and an outlet that can be connected to the inlet of the refrigeration circuit, and wherein the second compressor has a main inlet for receiving the refrigerant from the second outlet and an outlet that can be connected to the inlet of the refrigeration circuit.
The problems relating to the compressor size are even more pronounced with more recent liquefaction plants where the refrigerant is allowed to evaporate in four heat exchangers in series.
For this reason the invention further relates to an apparatus for compressing gaseous refrigerant for use in a refrigeration circuit of a liquefaction plant, which refrigeration circuit has an inlet for refrigerant at a refrigeration pressure, a first outlet for gaseous refrigerant at a low pressure, a second outlet for gaseous refrigerant at an intermediate pressure, a third outlet for gaseous refrigerant at a high pressure and a fourth outlet for gaseous refrigerant at a highxe2x80x94high pressure, which apparatus comprises according to the present invention a first compressor and a second compressor, wherein the first compressor has a main inlet for receiving the refrigerant from the first outlet, a side-inlet for receiving the refrigerant from the third outlet and an outlet that can be connected to the inlet of the refrigeration circuit, and wherein the second compressor has a main inlet for receiving the refrigerant from the second outlet, a side-inlet for receiving the refrigerant from the fourth outlet and an outlet that can be connected to the inlet of the refrigeration circuit.